Heat-hardened steel with excellent crashworthiness and method for manufacturing heat-hardenable parts using same

ABSTRACT

Disclosed are heat-hardened steel with excellent crashworthiness and a method for manufacturing heat-hardenable parts using the same. The heat-hardened steel according to the invention comprises, based on wt %; C: 0.12-0.8%; Cr: 0.01-2%; Mo: 0.2% or less; B: 0.0005-0.08%; Ca: 0.01 or less; Sb: 1.0% or less; and Ti and/or Nb: 0.2%; and the reminder being Fe and inevitable impurities. In addition, the heat-treatment hardening steel satisfies anyone of following conditions i)-iv), wherein condition i) comprises Si: 0.5-3%; Mn: 1-10% and Al: 0.05-2%; condition ii) comprises Si: 1% or less; Mn: 0.5-5%; Al: 0.1-2.5%; and Ni: 0.01-8%; condition iii) comprises Si: 0.5-3%; Mn: 1-10%; Al: 0.1% or less; and Ni: 0.01-8%; and condition iv) comprises Si: 0.5-3%; Mn: 1-10%; Al: 0.1-2.5%; and Ni: 0.01-8%.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of U.S. Ser. No. 14/115,516 filed Nov. 4, 2013, which is a National Phase application of No. PCT/KR2011/004785 filed on Jun. 30, 2011 and also which claims the benefit under 35 U.S.C. §119 of Korean Patent Application No. 10-2011-0064159 filed on Jun. 30, 2011 in the Korean Intellectual Property Office, the entirety of which disclosure is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a technology for manufacturing a high strength component using heat-treatment hardening steel, and more particularly, to heat-treatment hardening steel having high strength and crashworthiness after heat treatment and a method for manufacturing a heat-treatment hardening component using the same.

BACKGROUND ART

Recently, automobile components have been developed to be light in weight and to have high strength for improvement of fuel efficiency.

Recently, with the development of techniques for manufacturing automobile components, hot stamping has been developed. Hot stamping is a process of manufacturing a high strength component by quenching to form a martensite microstructure as soon as a material having a tensile strength of about 500 MPa and heated to about 900° C. is formed into a desired shape. Hot stamping may be used to produce high strength components having a tensile strength of 1000 MPa or more.

Steel for hot stamping comprises, in terms of % by weight (wt %), C: 0.23%; Si: 0.24%; Mn: 1.2%; Cr: 0.18%; Mo: 0.0025%; Al: 0.03%; Ti 0.035%; B: 0.002% and the balance of Fe and unavoidable impurities.

Steel having such a composition may exhibit a tensile strength of 490 MPa to 590 MPa and an elongation of 20% to 30% depending on process conditions. When the steel is heated to about 900° C., the steel may exhibit a tensile strength of 100 MPa to 200 MPa and an elongation of 50% to 60%, allowing easy forming. Then, when the steel is subjected to forming in dies and quenching, the formed steel has microstructures approaching full martensite, whereby a finished component has an ultra-high tensile strength of about 1470 MPa. The prepared component may have ultra-high strength and thus does not require a separate reinforcing material to enhance strength.

As such, hot stamping can facilitate weight reduction and reduce the number of welds through elimination of components such as a reinforcing material, thereby improving productivity while reducing manufacturing costs.

However, components manufactured by this process have a drawback in that such components have a low elongation of 6% to 7% due to microstructures approaching full martensite, which is advantageous for securing high strength.

Such low elongation causes brittleness failure of a component due to insufficient absorption of impact when external impact is applied thereto.

DISCLOSURE Technical Problem

An aspect of the present invention is to provide heat-treatment hardening steel that exhibits high ductility and toughness together with high strength through adjustment of alloy components after heat treatment, thereby providing improved crashworthiness.

Another aspect of the present invention is to provide a method for manufacturing a heat-treatment hardening component using the heat-treatment hardening steel.

Technical Solution

In accordance with one aspect of the present invention, heat-treatment hardening steel comprises, in terms of % by weight (wt %), C: 0.12˜0.8%; Cr: 0.01˜2%; Mo: 0.2% or less; B: 0.0005˜0.08%; Ca: 0.01 or less; Sb: 1.0% or less; at least one of Ti and Nb: 0.2% or less; components satisfying any one of the following compositions i) to iv); and the balance of Fe and unavoidable impurities.

By wt %,

i) Si: 0.5˜3%; Mn: 1˜10%; and Al: 0.05˜2%

ii) Si: 1% or less; Mn: 0.5˜5%; Al: 0.1˜2.5%; and Ni: 0.01˜8%

iii) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1% or less; and Ni: 0.01˜8%

iv) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1˜2.5%; and Ni: 0.01˜8%

The steel may have a layer selected from among an Al—Si plated layer, a galvanized layer, and a high temperature oxidation resistant coating layer on a surface thereof.

In accordance with another aspect of the present invention, a method for manufacturing a heat-treatment hardening component includes: (a) preparing a blank formed of the heat-treatment hardening steel as described above; (b) heating the blank; (c) hot-forming and quenching the heated blank in dies; and (d) performing post-treatment of a formed body formed in the (c) hot-forming and quenching.

In accordance with a further aspect of the present invention, a method for manufacturing a heat-treatment hardening component includes: (a) preparing a blank formed of the heat-treatment hardening steel as described above; (a′) performing primary-forming of the blank through cold working; (b) heating a primary formed body formed in the (a′) performing primary forming; (c) performing secondary-forming and quenching of the heated primary formed body in dies; and (d) performing post-treatment of a secondary formed body formed in the (c) performing secondary-forming and quenching.

Advantageous Effects

The heat-treatment hardening steel according to the present invention may provide a high strength, highly tough and highly ductile component having a tensile strength of 1000 MPa or more, a yield strength of 800 MPa or more, and an elongation of 10% or more through hot stamping. Accordingly, the component manufactured by the method according to the present invention may exhibit improved crashworthiness through high strength and excellent impact absorption capabilities.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic flowchart of a method for manufacturing a heat-treatment hardening component in accordance with one embodiment of the present invention.

FIG. 2 is a schematic flowchart of a method for manufacturing a heat-treatment hardening component in accordance with another embodiment of the present invention.

FIG. 3 shows a microstructure of a specimen prepared in Comparative Example 1.

FIG. 4 shows a microstructure of a specimen prepared in Example 1.

BEST MODE

The above and other aspects, features, and advantages of the present invention will become apparent from the detailed description of the following embodiments in conjunction with the accompanying drawings.

It should be understood that the present invention is not limited to the following embodiments and may be embodied in different ways, and that the embodiments are provided for complete disclosure and thorough understanding of the invention by those skilled in the art. The scope of the present invention will be defined only by the claims.

Hereinafter, heat-treatment hardening steel with excellent crashworthiness and a method for manufacturing a heat-treatment hardening component using the same according to the present invention will be described in detail.

Heat-Treatment Hardening Steel

Heat-treatment hardening steel according to the present invention comprises, by wt %, C: 0.12˜0.8%; Cr: 0.01˜2%; Mo: 0.2% or less; at least one of titanium (Ti) and niobium (Nb): 0.2% or less; B: 0.0005˜0.08%; and Sb: 1.0% or less.

In addition, the heat-treatment hardening steel satisfies at least one of the following compositions i) to iv):

By wt %,

i) Si: 0.5˜3%; Mn: 1˜10% and Al: 0.05˜2%;

ii) Si: 1% or less; Mn: 0.5˜5%; Al: 0.1˜2.5% and Ni: 0.01˜8%;

iii) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1% or less and Ni: 0.01˜8%; and

iv) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1˜2.5% and Ni: 0.01˜8%.

The heat-treatment hardening steel also comprises the balance of Fe and unavoidable impurities.

Now, the amounts and functions of the respective components included in the heat-treatment hardening steel according to the present invention will be described in more detail.

Carbon (C)

Carbon (C) is added to secure strength of steel. In addition, carbon serves to stabilize an austenite phase according to the amount of carbon enriched in the austenite phase.

Preferably, carbon is present in an amount of 0.12 wt % to 0.8 wt % based on the total weight of the steel. If the carbon content is less than 0.12 wt %, it is difficult to secure sufficient strength. On the contrary, if the carbon content exceeds 0.8 wt %, the steel can suffer from significant deterioration in toughness and weldability despite increase of strength.

Chromium (Cr)

Chromium (Cr) improves elongation through stabilization of ferrite crystal grains, and increases strength through stabilization of austenite by increasing the amount of carbon enriched in the austenite phase.

Preferably, chromium is present in an amount of 0.01 wt % to 2 wt % based on the total weight of the steel. If the chromium content is less than 0.01 wt %, the added chromium does not provide sufficient functions thereof. On the contrary, a chromium content of greater than 2 wt % makes it difficult to secure sufficient yield strength after heat treatment, and deteriorates wettability.

Molybdenum (Mo)

Molybdenum (Mo) is an effective element for enhancing strength of steel through precipitation strengthening and solid-solution strengthening. However, if the molybdenum content exceeds 0.2 wt %, the steel can suffer from deterioration in processibility.

Therefore, molybdenum is preferably present in an amount of 0.2 wt % or less based on the total weight of the steel.

Titanium (Ti), Niobium (Nb)

Titanium (Ti) and niobium (Nb) are carbonitride forming elements and sever to enhance strength of steel. However, if the total amount of titanium and niobium exceeds 0.2 wt %, the steel can suffer from deterioration in toughness. Therefore, titanium or niobium is preferably present in a total amount of 0.2 wt % or less based on the total weight of the steel.

Boron (B)

Boron (B) enhances strength of steel through quenching ability. Preferably, boron is present in an amount of 0.0005 wt % to 0.08 wt % based on the total weight of the steel. If the boron content is less than 0.0005 wt %, boron does not provide functions thereof. On the contrary, if the boron content exceeds 0.08 wt %, the steel can suffer from significant deterioration in toughness due to excessive increase in quenching ability.

Antimony (Sb)

Antimony (Sb) enhances coating properties of steel by preventing enrichment of silicon and manganese in grain boundaries. However, if the antimony content exceeds 1%, the steel can suffer from cracking and secondary work embrittlement.

Therefore, antimony is preferably used in an amount of 1% or less based on the total weight of the steel.

Silicon (Si), Manganese (Mn), Aluminum (Al), Nickel (Ni)

Through studies for long duration, the inventors of the present invention have found that silicon, manganese, aluminum and nickel enhance tensile strength, yield strength and elongation after heat treatment while satisfying at least one of the following compositions i) to iv).

By wt %,

i) Si: 0.5˜3%; Mn: 1˜10% and Al: 0.05˜2%

ii) Si: 1% or less; Mn: 0.5˜5%; Al: 0.1˜2.5% and Ni: 0.01˜8%

iii) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1% or less and Ni: 0.01˜8%

iv) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1˜2.5% and Ni: 0.01˜8%

In compositions i) to iv), silicon (Si) acts as a deoxidizer and enhances strength of steel through solid-solution strengthening. If the silicon content exceeds the range provided by each of compositions i) to iv), the steel can suffer from deterioration in weldability and coating properties. In addition, in the case of the compositions i), iii) and iv), if the silicon content is less than the proposed range, the steel can suffer from deterioration in weldability.

In compositions i) to iv), manganese (Mn) enhances strength of steel through austenite stabilization. If the manganese content is less than the proposed range in each of i)˜iv), the effect of stabilizing the austenite phase becomes insufficient. On the contrary, if the manganese content exceeds the range provided by each of compositions i)˜iv), there are problems of deterioration in weldability and toughness.

In compositions i) to iv), aluminum (Al) serves to prevent hydrogen embrittlement. If the aluminum content is less than the proposed range in each of i)˜iv), the effect provided by addition of aluminum can become insufficient. On the contrary, if the aluminum content exceeds the range provided by each of compositions i) to iv), aluminum forms excess inclusions, thereby deteriorating ductility and toughness of the steel.

In compositions ii) to iv), nickel (Ni) is advantageous in securing strength and toughness of steel. If the nickel content is less than the proposed range in each of compositions ii) to iv), the effect provided by addition of nickel can become insufficient. Conversely, if the nickel content exceeds the range provided by each of compositions ii) to iv), the effects provided by addition of nickel can become saturated, thereby significantly increasing manufacturing costs.

The heat-treatment hardening steel having the above composition according to the invention may be produced in forms of hot-rolled steel sheets, hot-rolled plated steel sheets, cold-rolled steel sheets, cold-rolled plated steel sheets, high temperature oxidation resistant coated steel sheets, and the like. Here, the heat-treatment hardening steel according to the present invention may have an Al—Si based coating layer, galvanized layer or high temperature oxidation resistant coating layer on a surface thereof in order to prevent decarburization and oxidation in a hot stamping process for fabrication of components described below. The Al—Si based coating layer and the galvanized layer are generally applied to cold-rolled plated steel sheets, without being limited thereto. In addition, the galvanized layer may be formed by various methods such as hot-dip galvanizing, hot-dip galvannealing, electro-galvanizing, and the like.

Here, when the heat-treatment hardening steel according to the present invention is a cold-rolled plated steel sheet, annealing may be performed at a temperature ranging from 650° C. to 850° C. If the annealing temperature is less than 650° C., it is difficult to achieve desired effects such as ductility improvement and the like even by annealing. Conversely, if annealing temperature exceeds 850° C., there is a high possibility of enrichment of silicon, manganese, and the like in grain boundaries even by addition of antimony, thereby causing deterioration in coating properties.

On the other hand, the heat-treatment hardening steel having the above composition according to the invention may have a tensile strength of 490 MPa to 980 MPa, a yield strength of 370 MPa to 600 MPa, and an elongation of 20% to 50% according to process conditions, that is, hot rolling, cold rolling, annealing, and the like. Although the heat-treatment hardening steel does not need to have these mechanical properties, heat-treatment hardening steel having these mechanical properties is advantageous in forming through hot stamping for fabrication of components.

In addition, the heat-treatment hardening steel having the above composition and mechanical properties according to the present invention may have a composite microstructure including martensite and retained austenite after heat treatment.

Further, the heat-treatment hardening steel having the above composition and mechanical properties according to the present invention may have a tensile strength of 1000 MPa or more, a yield strength of 800 MPa or more, and an elongation of 10% or more after heat treatment, since the retained austenite structure is included in the microstructure even after hot stamping.

Method of Manufacturing Heat-Treatment Hardening Component

FIG. 1 is a schematic flowchart of a method for manufacturing a heat-treatment hardening component in accordance with one embodiment of the invention.

Herein, the term “component” may refer to collision members of automobiles, without being limited thereto.

Referring to FIG. 1, the method for manufacturing a heat-treatment hardening component includes preparing a blank (S110), heating the blank (S120), forming/quenching (S130), and post-treatment (S140).

In operation of preparing a blank (S110), a blank is prepared from the heat-treatment hardening steel having the composition according to the present invention.

As described above, the heat-treatment hardening steel may have a tensile strength of 490 MPa to 980 MPa, a yield strength of 370 MPa to 600 MPa, and an elongation of 20% to 50%. In addition, considering blank heating (S120) and forming/quenching (S130) described hereinafter, the steel may have an Al—Si based coating layer, a galvanized layer, a high temperature oxidation resistant coating layer or the like formed on the surface thereof.

Next, in operation of heating the blank (S120), the blank is heated to a temperature suitable for hot stamping. Heating may be performed outside dies which will be used for hot stamping, that is, forming/quenching, and may be performed inside the dies after heating is performed to a predetermined temperature outside the dies.

The heating temperature may range from 700° C. to 1100° C. If the heating temperature is less than 700° C., austenite formation becomes insufficient, thereby causing insufficient strength after the operation of forming/quenching (S130). Conversely, if the heating temperature exceeds 1100° C., it is difficult to secure high ductility due to an insufficient fraction of the retained austenite after the operation of forming/quenching (S130), thereby causing deterioration of crashworthiness.

Next, in the operation of forming/quenching (S130), the blank heated in the dies is formed into a formed body having a predetermined shape, which in turn is subjected to quenching inside the dies to secure desired properties.

Quenching may be performed to a martensite transformation start temperature or less, for example, to a temperature ranging from about 80° C. to about 500° C., in order to secure the martensite fraction. In addition, quenching may be performed at a cooling rate of 10° C./sec to 300° C./sec. If the cooling rate is less than 10° C./sec, it is difficult to secure sufficient strength. Conversely, if the quenching rate exceeds 300° C./sec, it is difficult to secure toughness and ductility.

After forming/quenching, the formed body may have a composite microstructure comprising martensite and retained austenite. As a result, the formed body formed through forming/quenching may have a tensile strength of 1000 MPa or more, a tensile strength of 800 MPa or more, and an elongation of 10% or more.

In post treatment (S140), the formed body formed through forming/quenching is subjected to laser processing to perform trimming, piercing, and the like.

FIG. 2 is a schematic flowchart of a method for manufacturing a heat-treatment hardening component in accordance with another embodiment.

Referring to FIG. 2, the method for manufacturing a heat-treatment hardening component includes blank preparation (S210), cold working (S215), heating the blank (S220), forming/quenching (S230), and post treatment (S240).

In the embodiment shown in FIG. 2, the method further includes cold rolling (S215). In operation of cold rolling (S215), the blank is subjected to primary forming through cold working. In this case, during primary forming through cold working, a primary formed body is prepared through forming, trimming, piercing, and the like. Thus, in post treatment (S240), laser processing is performed on a portion of a secondary formed body, which is subjected to secondary forming (S230) through forming/quenching within dies.

EXAMPLES

Next, the present invention will be described in more detail with reference to examples. Here, the following examples are provided for illustration only and should not be construed in any way as limiting the present invention.

Descriptions of details apparent to those skilled in the art will be omitted.

1. Preparation of Specimen

In order to observe heat-treatment hardening properties of steel according to alloy compositions, specimens of Examples 1 to 4 and Comparative Example 1 having compositions as listed in Table 1 and mechanical properties before heat treatment as listed in Table 2 were heated to 900° C., left for 5 minutes, and cooled to 100° C. at an average cooling rate of 50° C./sec.

TABLE 1 (Unit: wt %) C Si Mn Cr Mo Al Ti Nb B Ni Sb Comparative 0.229 0.238 1.19 0.183 0.0025 0.03 0.036 — 0.002 — — Example 1 Example 1 0.3 1.0 7.5 0.3 0.01 1.5 0.05 — 0.003 — 0.8 Example 2 0.3 0.4 3.0 0.3 0.01 2.0 0.05 0.01 0.005 2.0 0.8 Example 3 0.4 1.5 5.5 0.2 0.01 0.05 0.05 0.05 0.003 3.0 0.8 Example 4 0.4 1.7 6.0 0.2 0.01 2.0 — 0.10 0.002 3.0 0.8

2. Mechanical Properties

Table 2 shows mechanical properties of the specimens of Examples 1 to 4 and Comparative Example 1 before and after heat treatment.

TABLE 2 Before heat treatment After heat treatment Tensile Yield Tensile Yield Fraction of strength strength Elongation strength strength Elongation retained (MPa) (MPa) (%) (MPa) (MPa) (%) austenite (%) Comparative 510 380 25 1470 840 6.0   <1% Example 1 Example 1 515 383 26 1291 1136 15.0  3~5% Example 2 520 382 24 1302 1124 14.8  5~15% Example 3 710 491 22 1884 1138 15.1 10~40% Example 4 723 490 21 1817 1054 14.7 30~60%

Referring to Table 2, the specimens of Examples 1 to 4 and Comparative Example 1 exhibited similar mechanical properties before heat treatment.

However, after heat treatment, the specimen of Comparative Example 1 had a low elongation of 6% despite very high tensile strength. On the contrary, although the specimens of Examples 1 to 4 had slightly lower tensile strength than the specimens of Comparative Example 1, these specimens had an elongation of about 15% and exhibited relatively high yield strength.

Accordingly, upon application of external impact, the specimen of Comparative Example 1 can suffer from brittleness failure due to low yield strength and elongation as compared with tensile strength, whereas the specimens of Examples 1 to 4 can sufficiently absorb the impact due to relatively high yield strength and elongation.

In addition, in order to measure the fraction of retained austenite, various tests such as microscopic observation, magnetic measurement, X-ray diffraction analysis, and the like were performed. As a result, although the specimens of Examples 1 to 4 have different values according to the measurement methods, these specimens include retained austenite in an area fraction of at least 1% or more.

However, the specimen of Comparative Example 1 included retained martensite in an area fraction of less than 1% even by any measurement methods, and thus had a full martensite microstructure.

Difference in physical properties after heat treatment between the specimens of Examples 1 to 4 and Comparative Example 1 can be confirmed through difference in final microstructure.

FIG. 3 shows the microstructure of a specimen prepared in Comparative Example 1, and FIG. 4 shows the microstructure of a specimen prepared in Example 1.

Referring to FIG. 3, the specimen of Comparative Example 1 had a microstructure approaching full martensite. On the other hand, referring to FIG. 4, it can be seen that the specimen of Example 1 includes retained austenite (y) in addition to martensite.

By such microstructures, the specimen of Comparative Example 1 can have a very low elongation despite very high yield strength, whereas the specimen of Example 1 can have high elongation.

Although some embodiments have been disclosed herein, it should be understood that these embodiments are provided for illustration only and various modifications, changes, alterations and equivalent embodiments can be made without departing from the scope of the present invention. Therefore, the scope and sprit of the invention should be defined only by the accompanying claims and equivalents thereof. 

1. A method for manufacturing a heat-treatment hardening component, comprising: (a) preparing a blank formed of heat-treatment hardening steel, the heat-treatment hardening steel comprising: by wt %, C: 0.12˜0.8%, Cr: 0.01˜2%, Mo: 0.2% or less, B: 0.0005˜0.08%, Ca: 0.01 or less, Sb: 1.0% or less, at least one of Ti and Nb: 0.2% or less, components satisfying anyone of the following compositions i) to iv), and the balance of Fe and unavoidable impurities; By wt %, i) Si: 0.5˜3%; Mn: 1˜10% and Al: 0.05˜2% ii) Si: 1% or less; Mn: 0.5˜5%; Al: 0.1˜2.5% and Ni: 0.01˜8% iii) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1% or less and Ni: 0.01˜8% iv) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1˜2.5% and Ni: 0.01˜8%, (b) heating the blank; (c) hot-forming and quenching the heated blank in dies; and (d) performing post-treatment of a formed body formed in the (c) hot-forming and quenching.
 2. The method according to claim 1, wherein the (b) heating is performed by heating the blank to a temperature of 700° C. to 1100° C.
 3. The method according to claim 1, wherein, in the (c) hot-forming and quenching, quenching is performed by cooling the heated blank in the dies at a rate of 10° C./sec to 300° C./sec to a martensite transformation start temperature or less of the heat-treatment hardening steel.
 4. The method according to claim 1, wherein the heat-treatment hardening steel has at least one layer selected from an Al—Si based coating layer, a galvanized layer and a high temperature oxidation resistant coating layer on a surface thereof.
 5. A method for manufacturing a heat-treatment hardening component, comprising: (a) preparing a blank formed of heat-treatment hardening steel, the heat-treatment hardening steel comprising:, by wt %, C: 0.12˜0.8%, Cr: 0.01˜2%, Mo: 0.2% or less, B: 0.0005˜0.08%, Ca: 0.01 or less, Sb: 1.0% or less, at least one of Ti and Nb: 0.2% or less, components satisfying anyone of the following compositions i) to iv), and the balance of Fe and unavoidable impurities; By wt %, i) Si: 0.5˜3%; Mn: 1˜10% and Al: 0.05˜2% ii) Si: 1% or less; Mn: 0.5˜5%; Al: 0.1˜2.5% and Ni: 0.01˜8% iii) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1% or less and Ni: 0.01˜8% iv) Si: 0.5˜3%; Mn: 1˜10%; Al: 0.1˜2.5% and Ni: 0.01˜8%, (a′) performing primary-forming of the blank through cold working; (b) heating a primary formed body formed in the (a′) performing primary forming; (c) performing secondary-forming and quenching of the heated primary formed body in dies; and (d) performing post-treatment of a secondary formed body formed in the (c) performing secondary-forming and quenching.
 6. The method according to claim 5, wherein the (b) heating is performed by heating the blank to a temperature of 700° C. to 1100° C.
 7. The method according to claim 5, wherein, in the (c) performing secondary-forming and quenching, quenching is performed by cooling the heated blank in the dies at a rate of 10° C./sec to 300° C./sec to a martensite transformation start temperature or less of the heat-treatment hardening steel.
 8. The method according to claim 5, wherein the heat-treatment hardening steel has at least one layer selected from an Al—Si based coating layer, a galvanized layer and a high temperature oxidation resistant coating layer on a surface thereof. 